Currently the conventional technique used to evaluate bone mineral density and therefore risk of fragility fracture is a DXA scan (dual energy X-ray absorptiometry). However, DXA alone cannot accurately predict an individual's risk of osteoporotic fracture. Broadly speaking bone comprises a dense outer layer of so-called cortical bone, within which is found trabecular bone, which has a more spongy structure and which is more porous. The outer, cortical bone is the most important in conferring resistance to fracture/breakage and improved techniques for investigating the thickness of this structure would be useful, particularly as exercise/drugs can be used to improve bone strength. General background prior art can be found in: GB2379113A, WO2004/086972, and WO2005/091222.
The exponential increase in hip fractures with ageing imposes a substantial and increasing health burden. They are the most common reason for acute orthopaedic admission in older people and their incidence is projected to rise worldwide from 1.7 million in 1990 to 6.3 million in 2050. Current imaging methods for predicting an individual's hip fracture risk use bone mineral density (BMD) estimates which are specific but have low sensitivity. Regional structural weakness in the aged hip may be causal over and above the risk of falls and there have been numerous calls to develop novel three-dimensional (3D) methods of assessing hip structure, most notably with multi-detector computed tomography, MDCT.
The anatomical distribution of cortical bone in the aged proximal femoral metaphysis is critical in determining its resistance to fracture, with trabecular bone playing a lesser role. Compressive cracking of cortical bone of the superior trochanteric fossa and neck is often the first stage of fracture. Current MDCT studies are anatomically limited by set regions of interest and technically limited by thresholding errors caused by relatively low resolution data sets. Ideally, we would like to be able to map the cortical bone thickness to a high degree of accuracy, from a clinical resolution data set, over the entire proximal femur. Since bone anabolic drugs are capable of stimulating new bone formation at structurally important sites, such a technique might help not only with identification of weak areas but also in assessing treatment response.
Thin laminar structures such as the femoral cortex are not accurately depicted in clinical CT because of the images' limited spatial resolution. This typically results in an underestimate of density and an overestimate of thickness. A number of studies have investigated the influence of the imaging system's in-plane point spread function (PSF), while Prevrhal et al [Prevrhal, S., Fox, J. C. Shepherd, J. A., Gennant, H. K., January 2003, Accuracy of CT-based thickness measurement of thin structures: Modelling of limited spatial resolution in all three dimensions, Medical Physics 30(10, 1-8] go further in quantifying the effects of the out-of-plane PSF as well. Such studies explain why straightforward thickness estimation techniques, such as those based on thresholding or the 50% relative threshold method, are unreliable when the structure is thin compared with the imaging system's PSF. The consensus is that, with typical PSFs from normal bore, clinical CT scanners, straightforward thickness measurements start to become inaccurate below around 2.5 mm, with errors exceeding 100% for sub-millimeter cortices.
More sophisticated thickness estimation techniques are capable of superior accuracy and precision. The classical approach to image deblurring is to assume models of both the object being scanned and the image formation process, then attempt to fit these models to the data. Often, this inverse problem is ill-posed, in that no unique set of model parameters explains the data, especially in the presence of imaging noise. In such circumstances, prior knowledge may be used to constrain some of the model parameters. Such an approach was used by Streekstra et al [Streekstra, G. J., Strackee, S. D., Maas, M., ter Wee, R Venema, H. W., September 2007, Model-based cartilage thickness measurement in the submillimeter range, Medical Physics 34 (9), 3562-3570] to estimate the thickness of cartilage layers in the sub-millimeter range, well below the nominal width of the CT system's PSF. In this instance, the model parameters relating to the PSF were fixed, using information obtained by scanning a suitable phantom.
A typical CT slice has a thickness of order 3 mm; typical in-plane blur is of order 1.5-2 mm (Gaussian full width at half maximum). The thresholding type techniques described above for cortical thickness estimation work well when the cortical thickness is large, for example, 6-7 mm, but do not work well for cortical bone thickness in the range 0-3 mm. However risk of fracture is typically associated with a cortical bone thickness of less than 2 mm.
There is therefore a need for improved techniques for estimating the thickness of a tissue structure in particular cortical bone from tomographic imaging data. Work on this problem has produced some surprising results.